There have been many different types and kinds of electronic locks and electronic protection devices for helping to protect and secure valuables. Such electronic devices must operate effectively and reliability and must be immune to electrically polluted environments including electrostatic discharges and power surges. Such devices must also be immune to the effects of tampering in order to prevent unwanted and unauthorized access to the secured valuables.
In order to protect electronic devices from sudden and unexpected power surges many electronic locks include some form of electronic protection that clamps transients while passing through different EMC homologation. Such protective devices are typically called transient voltage suppressors or simply a TVS.
Such suppressor devices are also utilized to help prevent unwanted and unauthorized access to small component, battery powered electronic locks. In this regard, the typical actuating device in a small component, battery powered electronic lock is usually a transistor driven device, such as small electronic motor or a solenoid, that operate at a low current of a half an ampere or less. Operationally then, the transistor driver is biased in a non-conductive state to conserve power and as such, is held in the non-conductive state by a constant bias voltage of about 5 volts. The TVS then helps clamp the voltage presented to the transistor driver at the desired bias voltage so an over voltage avalanche condition is avoided thereby allowing the solenoid to be actuated for opening the lock.
While a TVS may be suitable for clamping a transient power condition, a TVS is not designed to be immune from continuous overvoltage and or current out of tolerance conditions. Thus, the use of a TVS by itself is not immune to the effects of tampering.
More particularly, in the typical small component, battery powered electronic lock, power consumption must be minimal, which in turn dictates, that surge protection devices as well as lock actuating devises must also have low power ratings. Thus, application of a high constant current to a surge protection device will cause such a TVS to heat rapidly and explode due to rapid thermal expansion. Such a failure may under certain circumstances allow a theft to gain access to the valuables secured by the lock. In this regard, without a clamped bias voltage, a driver transistor may be subjected to an overvoltage avalanche condition as earlier mentioned.
In an attempt to overcome the problems associated with continuous overvoltage and or current out of tolerance conditions, some electronic devices and locks have incorporated in their protective circuits an in series slow blow fuse. A slow blow fuse is a one use passive device that must be replaced when it fails due to a continuous overvoltage and or current out of tolerance condition.
For example, application of a 2.times.9 volt overvoltage in one well known electronic lock causes the instantaneous failure of its safety fuse. Thus, while the fuse served its purpose by protecting the other electronic components within the lock, the lock is rendered useless until the fuse is replaced. Moreover, this type of tampering cannot be detected and may cause user concerns relative to the reliability of the lock or fuse circuit design should it happen on a regular basis.
Thus, while the addition of a slow blow fuse may be satisfactory for helping to protect a TVS from a high current, the slow blow fuse has not proven entirely satisfactory when employed for helping to detect lock tampering and sabotage.
Lock tampering, unlike lock sabotage, is an attempt to cause some specific component within the lock to fail by the application of an overvoltage or out of tolerance current condition so access through the lock to the otherwise secured protected valuables can be achieved.
From the perpetrator point of view, the tampering process should be undetected by the owner of the protective device. In this regard, the perpetrator desire no detection since the owner will be less likely to consider tampering as the means of achieving access to the protected goods. From the owner point of view, the lost of valuables will be unfortunate, but if detection is possible, the owner may be able to take additional precautionary steps, particularly if access was achieved by tampering with the electronic security device.
Unfortunately for the owner, clever unsuccessful tampering is difficult, if not impossible to detect. Moreover, should such tampering cause the protective fuse to fail while not preventing access through the lock, consumer confidence in electronic protective devices can be eroded.
In order to at least help deter the detrimental effects of sabotage and tampering, many electronic surge protection circuit utilize a resettable fuse, such as a POLYSWITCH (a registered trademark owned by Raychem Corporation located in Menlo Park, Calif.), that operates on the basis of a positive temperature coefficient. The resettable fuse in its normal state of operation has a low resistance. However, as soon as an out of tolerance current condition occurs, the resettable fuse will begin to heat and simultaneously will increase its resistance. In this regard, by increasing the resistance, the current passing to the other electronic components within the lock is limited to a safe level. Thus, unlike the slow blow fuse, the resettable fuse will not open circuit, but instead will merely limit the current passing to the other electronic components.
While a POLYSWITCH approach may help alleviate consumer concerns about lock and fuse reliability, the use of a resettable fuse in small component security locks has not proven entirely satisfactory with respect to tampering detection. In this regard, a resettable fuse has a slow response time and thus, is not effective when attempting to protect a TVS from thermal failure.
More particularly, as a resettable fuse is a thermally sensitive device, it must be designed within a given lock, to support certain maximum current values over a particular temperature range. Thus for example, in a typical small component electronic lock with an operating current of about 0.5 amperes, such a fuse would support a significantly greater current capacity at a 20.degree. C. temperature as opposed to an 80.degree. C. temperature. Design criteria would therefore dictate in a small component electronic lock that the resettable fuse pass at least 0.25 amperes but less than 1.5 amperes at a maximum operating temperature of 20.degree. C.
A sophisticated thief knowing the operational characteristics of the POLYSWITCH in a 3 watt electronic lock would be able to easily determine that the maximum constant current the POLYSWITCH would support would be 0.25 amperes (3 watts/12 volts=0.25 amperes). Therefore the thief would be able to apply a low current of 0.25 amperes to the surge protection circuit, i.e. the TVS, without affecting the POLYSWITCH. As the TVS is not designed however, to handle such constant currents, a 0.25 amperes constant current at a sufficiently high voltage, would be sufficient to slowly cook the protective TVS until it fails in an open circuit condition.
Once the thief causes the TVS failure, the thief would then reduce the applied current (in order to protect the low wattage solenoid) and apply a sufficiently high voltage to now cause the driver transistor to fail in an avalanche condition. In the avalanche condition, the thief would then be able to decrease the voltage to a safe operating level followed by increasing the applied current to a sufficient amperage to activate the solenoid.
Therefore, it would be highly desirable to have a new and improved small component electronic lock that includes a tamper protection circuit that is effective against both short duration transient conditions and long duration overvoltage and or out of tolerance currents in order to protect the lock protected valuable from unauthorized access.